We have noted that IV DMT elicits robust elevations in blood levels of beta-endorphin compared to placebo. This rise in BE precedes a similarly robust increase in prolactin levels. We would like to determine the effects of blockading endogenous opioids using oral nalhexane before DMT administration. We will assess effects of opioid blockade on the subjective, opioid-like effects of DMT, as well as effects on BE and prolactin levels.